Electronic devices are ubiquitous in modern society, prompting technological development in related or peripheral fields, such as transistor materials and methods for their manufacture. Current methods for producing functional devices from inorganic materials typically employ sequential deposition and etching of selected semiconducting, conducting, and insulating materials. These steps are performed using multiple photolithography and vacuum deposition steps, such as physical or chemical vapor deposition, which contribute to their high manufacturing costs. Methods that reduce or eliminate one or more of these steps would benefit commercial production of electronic devices.
Thin films conventionally are deposited by a CVD (Chemical Vapor Deposition) method and a sputtering method. Silicon films, such as amorphous silicon films, and poly-silicone films, typically are used for the semiconductor films and have been formed by thermal CVD, plasma CVD or photo-CVD using monosilane or disilane gas. Thermal CVD generally has been used for depositing poly-silicon films (J. Vac. Sci. Technology, Vol. 14, p 1082 (1977)). Plasma CVD has been used for depositing amorphous silicon (Solid State Com., Vol. 17, p 1193 (1975)).
Silicon films formed by CVD have experienced manufacturing complications. For example, the yield is low due because the manufacturing apparatus becomes contaminated and side products are formed. Moreover, a silicon film having a uniform thickness cannot be deposited on a substrate having a rough surface, since a gaseous starting material is used. The substrate also must be heated, and productivity is low because of unacceptably slow film deposition rates. Finally, complicated and expensive microwave generators and evacuation apparatuses are necessary for plasma CVD deposition.
Inkjet printed organic materials are known. Sirringhaus et al., for example, fabricated all-polymer thin film transistors using a combination of inkjet printing and spin-coating. Sirringhaus, H., Kawase, T., Friend, R. H., Shimoda, T., Inbasekaran, M., Wu, W., Woo, E. P., “High-resolution inkjet printing of all-polymer transistor circuits,” Science, 290, 2123-2126 (2000). A mobility of 0.02 cm2/V·sec was achieved by spin-coating a semiconducting polymer channel layer. Researchers at IBM developed a one-step synthetic process for making a soluble pentacene precursor. Afzali A., Dimitrakopoulos, C. D., Breen, T. L., “High-performance, solution-processed organic thin film transistors from a novel pentacene precursor,” JACS Comm. 124, 8812-8813 (2002). The first inkjet-printed pentacene transistor was fabricated in 2003 with a mobility of 0.02 cm2/V·sec and a current on-to-off ratio of 105. Volkman S. K., Molesa, S., Mattis, B. Chang, P. C., Subramanian, V., “Inkjetted organic transistors using a novel pentacene precursor,” Mat. Res. Soc. Symp. Proc. 769, H11.7.1/L12.7.1-H11.7.6/L12.7.6 (2003). Arias et al. reported an inkjet-printed TFT using a polythiophene semiconductor channel having a field effect mobility of 0.1 cm2/V·s, and a current on-to-off ratio of 107. Arias, A. C. et al., “Polymer transistor display backplanes: high performance inkjet printed devices,” Abstract of papers, 229th ACS National Meeting, San Diego, Calif., United States (2005). Recently, Kawasaki et al. reported an organic TFT that was made using an inkjet-printed pentacene channel layer having a mobility of 0.15 cm2/V·s (the highest value for all reported inkjet printed TFTs) and a current on-to-off ratio of 105. Kawasaki, M. et al., “Printable organic TFT technologies for FPD applications,” Proceedings of SPIE—The International Society for Optical Engineering 5940 (Organic Field-Effect Transistors IV) (2005).
Solution-based printing methods offer one approach to address this issue. To date, very few inorganic materials have been printed onto substrates to make electronic devices. Most published reports concern printing metal nanoparticle solutions for metallization. For example, copper nanoparticle solutions were inkjet printed for source/drain metallization of silicon-based transistors. Hong, C. M., Wagner S., “Inkjet printed copper source/drain metallization for amorphous silicon thin-film transistors,” IEEE Electron Device Lett. 21(8), 384-386 (2000). Silver and gold nanoparticle solutions have been used for inkjet printing active microelectromechanical systems (MEMS). Fuller, S. B., Wilhelm, E. J., Jacobson, J. M., “Ink-jet printed nanoparticle microelectromechanical systems,” Journal of microelectromechanical systems 11(1), 54-60 (2002). Ridley et al. fabricated a thin film transistor having a mobility of 1 cm2/V·s and a current on-to-off ratio of 3.1×104 by casting CdSe thin films from a precursor solution of cadmium selenide nanocrystals using a micro-pipette. Ridley, B. A., Nivi, B., Jacobson, J. M., “All-inorganic field effect transistors fabricated by printing,” Science 286(5440), 746-749 (1999).
Transparent conducting oxides (TCOs), like zinc oxide, tin oxide, and indium tin oxide, are important for a plethora of optical and electrical applications. For example, such materials are useful for making flat-panel displays, organic light-emitting diodes, electromagnetic shielding, and electrochromatic windows. See, for example, MRS Bulletin, Transparent Conducting Oxides, 25(8), 22-65 (2000); and Chopra, K. L., Major, S., Pandya, D. K., “Transparent conductors-a status review,” Thin Solid Films 102, 1-46 (1983). More recently, conductive oxide materials have been used as channel materials for thin film transistors. See, for example, Nomura, K., Ohta H., Takagi A., Kamiya T., Hirano M., Hosono H., “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors,” Nature 432, 488-492 (2004); and Nomura, K., Ohta H., Ueda K., Kamiya T., Hirano M., Hosono H., “Thin-film transistor fabricated in single-crystalline transparent oxide semiconductor,” Science 300(5623), 1269-1272 (2003).
Methods for solution processing materials for electronic device applications also have been patented. For example, Ostergard, U.S. Pat. No. 6,946,677, entitled “Pre-Patterned Substrate for Organic Thin Film Transistor Structures and Circuits and Related Method for Making Same,” concerns forming a desired circuit configuration in the surface of a substrate, thereby pre-patterning the area to receive material useful for forming an organic thin film transistor (OTFT) structure and interconnecting conductive paths. According to the '677 patent, the “OTFT material is deposited in the pre-patterned area using printing techniques such as inkjet printing.”
Weng et al., U.S. Pat. No. 6,927,108, also concerns solution processing thin-film materials for forming transistors. The '108 patent discloses forming “conductive solution-processed thin film material contacts, semiconductor solution-processed thin film material active regions, and dielectric solution-processed thin film material isolations in a sequence and organization to form a solution-processed thin film structure capable of transistor operation.” Additional structure is formed by laser ablation “in one or more of the conductive solution-processed thin film material contacts, the semiconductor solution-processed thin film material active regions and the dielectric solution-processed thin film material isolations to pattern or complete patterning of a material being selectively ablated.” The method may involve “depositing drain and source conductive solution-processed thin film material and depositing gate conductive material solution-processed'thin film material” by inkjet printing conductive solution-processed thin film material. The '108 patent states that:
categories of solution-processed thin films include organic thin films and polymer thin film categories. The majority of the solution-processed materials that can be formed into thin films are the conductive polymers, semiconductive polymers and dielectric polymers. However, a solution-processed material may also be a precursor of small organic molecular material that is soluble in a solvent. One example is the pentacene precursor that is soluble in chloroform. It can be spin-coated to form a thin film and then heated to reduce to pentacene at temperatures of ≈200° C. Pentacene is an organic semiconductor but is not a polymer.
The '108 patent also states that “there may be inorganics that may be solution-processed to form thin films.” However, no species of inorganic material appears to be identified by the '108 patent, nor is any detail provided by the '108 patent that would enable a person of ordinary skill in the art to solution-process an inorganic material to form electronic devices.